The long-term goal is to understand how calcium is used to control cell growth and differentiation, from signal perception, to gene expression, to changes in cell structure and physiology. The focus is on a novel family of calcium pumps identified in plants (ACAs). These pumps are most closely related to the plasma membrane calcium pumps in animals (PMCAs). A genetic disruption has been obtained for each of the 14 calcium pumps in the model plant Arabidopsis. Homozygous disruptions of ACA9 (4 alleles) show a semi-sterile phenotype. Three specific aims are proposed: 1) Determine the developmental defects responsible for partial sterility. The hypotheses being tested are that a plasma membrane calcium pump is required for (i) normal pollen-tube growth and (ii) the delivery of sperm to egg cells. 2) Determine the regulatory features of ACA9 that are critical to its functions in fertilization. The hypothesis being tested is that a key event in fertilization involves a calcium signal that activates ACA9-mediated calcium-efflux. 3) Identify intragenic and extragenic mutations that modify the biochemical and biological functions of ACA9. The purpose is advance the understanding of ACA9's structure, regulation and function, as a paradigm for calmodulin-regulated calcium pumps in plants and animals. ACA9 is the first calcium transporter to be identified with genetic evidence for a role in either pollen growth or fertilization. In addition, aca9 is the first gametophytic mutation to be identified at the gene level that disrupts pollen-tube/ovule interactions. Our proposed genetic screens with plants and yeast provide an innovative approach to dissect the structure and biological functions of a calmodulin-regulated calcium pump. Our studies include a comparison of ACA9 with a human pump, PMCA4, which has been implicated in fertilization in animals.